A Facebook user has sent police to save a young woman being gang raped at gunpoint after her three-hour sexual assault was live-streamed online.

Josefine Lundgren, 21, contacted Swedish police after stumbling upon a Facebook livestream of a woman being raped by three men on Sunday.

Ms Lundgren said she saw the men pull their alleged victim’s clothes off and lay on top of her, with social media users making comments on the thread during the attack such as “three against one haha”, Exressen reported.

The 21-year-old said she was disgusted that at least 60 people sat by idly and watched the vicious assault, which included the men taunting and laughing at their victim, while she was the only one who called police.

Three men have been arrested after allegedly live-streaming the rape of a young woman on Facebook. (Source: Facebook) ()

“This is insane. It is only sick people who do such a thing. More people should care and act,” she told Expressen.

Ms Lundgren continued to watch the live feed until police arrived on the scene and stopped the transmission.

The 21-year-old has spoken to the victim of the alleged attack to explain she had her support.